Please refer to FIG. 1 and FIG. 2, which show a passive radio frequency identification system (RFID) of prior arts, that having an inducing side tag 11 and a reading side reader 12. Each side have an inductance is adapted for proceeding the electromagnetic inter-inducing effect and the transmitting behavior of energy and information. The reading side reader 12 of the RFID system may provide sufficient space and cost to arrange a stable DC power source, that to make the reading side reader 12 emit stable high frequency resonance carrier wave and also have demodulating function. The inducing side tag 11 due to the limitation of available area and weight, let the inducing side tag 11 is unable to provide direct and stable DC voltage source for actuation, but a stable system timing clock signal indeed needs a stable DC voltage source to act as a operating voltage.
In tradition, an antenna 21 (exactly is an inductance) at the inducing side tag 11 of a passive RFID is applied as the power source. The antenna of inducing form may be induced the constant radio frequency resonance signal that emitted from the resonance circuit of the reading side reader 12. The radio frequency is to be acted as the source of AC/DC rectifier at the inducing side tag 11 of a passive RFID. Through a full-wave or half-wave rectifier 22 comprised by four or two diodes, a DC power conversion is made. Again, through a simple RC low-pass filter 23, a DC voltage source V1 of approximately stable state is taken out to be act as the operating voltage source for actuating the oscillating circuit 25 and the all circuits at the inducing side tag 11 (please refer to FIG. 4). According to the supplied DC voltage source V1, the oscillating circuit 25 at the inducing side tag 11 oscillates a timing clock signal. Through a simple data coding generation and modulation circuit, the radio frequency include the modulated data information is sent out via the antenna. At the reading side reader 12, through the process of the detecting circuit and demodulation circuit, the weak carrier wave signal variation will be received and demodulated by the reading side reader 12 to an accurate data information signal. Again, through a micro-control unit (MCU), a preset sound or voice may be sent out to accomplish a signal-transmitting objective.
In the prior RFID system, the inducing side tag 11 itself is applied in a suspending state and is not grounded. Limited by the considering factors of the space and cost of the inducing side tag 11, the RC-filtering circuit 23 is unable to provide sufficient filtering effect, the high frequency ripple waveform is generated on the DC bias voltage. It directly influenced the stability of the system timing clock.
In a common prior RFID system, both of the inducing side tag 11 and the reading side reader 12 are not applied at a fixed distance. And, the operating voltage of the inducing side tag 11 is applies the parallel inductance of the inducing side tag 11 and the reading side reader 12 to generate an inducing voltage V1 as shown in FIG. 6, The magnitude of the inducing voltage V1 will be influenced by the distance between the inducing side tag 11 and the reading side reader 12. Due to the inter-induction effect, when both are closer, the inducing voltage V1 will become larger. Vice versa, when both are farther, the inducing voltage V1 will become smaller. When the inducing side tag 11 is moved toward the reading side reader 12, the inducing voltage V1 is raised. However, when the inducing side tag 11 is moved away the reading side reader 12, the inducing voltage V1 will be lowered down. Because the inducing voltage V1 is taken as the operating voltage of the oscillating circuit 25, the variation of the inducing voltage V1 will influence the timing clock signal oscillated by the oscillating circuit 25.
In addition, in the circuit of common inducing side tag 11, there will be a voltage limiter 24 to prevent the circuit from damaging because the inducing voltage is too large to exceed the physical enduring ability of the semiconductor elements when the inducing side tag 11 is too close to the reading side reader 12.
IN the integration circuit process, some feature is limit by the semiconductor process parameter and basic element architecture. For example, the high frequency noise through the IC's common substrate is to penetrate and to influence the entire system's operation. Therefore, the originally stable timing clock signal to any unexpected state, and to cause the whole system function fail.